1. Field of the Invention
The present invention relates to a crystal pulling method and apparatus employing, for example, the Czochralski method (the CZ method), and particularly to an improvement on a technique for mechanically holding a heavy single-crystal in order to pull the crystal safely and reliably, which improvement reduces variations in the diameter of a growing crystal at a point of time when the crystal is held, thereby enabling accurate growth of the crystal.
2. Description of the Related Art
Regarding a method of pulling a single crystal from a melt of a semiconductor material, such as silicon, contained in a crucible, the present applicant have proposed an improved pulling technique as described in, for example, Japanese Patent Application No. 8-212055.
Conventionally, a single crystal is pulled in a manner as shown in FIG. 5: a seed crystal 53 held by a seed chuck 52 at the lower end of a wire 51 is brought into contact with material melt contained in an unillustrated crucible, and the seed crystal 53 is pulled by a seed chuck lifting mechanism 54 in order to grow a single crystal C below the seed crystal 53. In order to improve this conventional manufacturing method, the technique disclosed in Japanese Patent Application No. 8-212055 employs an apparatus having the following structure: a retaining case 57 is mounted on a slider 56 which is vertically moved by an unillustrated slider moving mechanism; the seed chuck lifting mechanism 54 for winding the wire 51 is provided within the retaining case 57; and a lifting jig 60 composed of paired members is provided below the retaining case 57. The lower ends of the paired members of the lifting jig 60 are opened and closed through operation of cylinder units 58. At the beginning of pulling, the single crystal C is pulled by the seed chuck 52, and pulling by the seed chuck 52 is switched to pulling by the lifting jig 60 when the crystal C has grown to become heavy.
That is, initially, with the slider 56 fixed at a certain position, the seed crystal 53 held by the seed chuck 52 is brought into contact with the material melt, and subsequently the seed chuck 52 is pulled by the seed chuck lifting mechanism 54. As a result, as shown in FIG. 5A, a neck portion 59, a corrugated portion Ck, and the single crystal C are formed below the seed crystal 53.
When the corrugated portion Ck rises to a predetermined position shown in FIG. 5B, the lifting jig 60 is operated as shown in FIG. 5C such that the lower end portions of the paired members of the lifting jig 60 approach the smaller-diameter portion of the corrugated portion Ck with a clearance remaining between the lower end portions of the paired members of the lifting jig 60 and the smaller-diameter portion of the corrugated portion Ck. Subsequently, as shown in FIG. 5D, the slider 56 is raised, while the seed chuck lifting mechanism 54 is operated in reverse in order to lower the single crystal C relative to the slider 56, such that the larger-diameter portion of the corrugated portion Ck comes into contact with and sits on the lifting jig 60.
At this time, control is performed such that the sum of the rising speed Vb of the slider 56 and the rising speed Va of the seed chuck 52 relative to the slider 56 becomes identical to the previous rising speed of the seed chuck 52 as measured when solely the seed chuck 52 was engaged in pulling. Also, when a predetermined portion of the entire load of the single crystal C has shifted to the lifting jig 60, the seed chuck lifting mechanism 54 is stopped, as shown in FIG. 5E, so that the pulling operation is performed by only the lifting jig 60; i.e., only the slider 56 is raised.
Since the lifting jig 60 contacts the corrugated portion Ck of the crystal not in a lateral direction but in the vertical direction, the crystal can be pulled safely and reliably without generation of dislocation in the crystal.
In this technique, when the pulling by the seed chuck 52 is switched to the pulling by the lifting jig 60, as shown in FIG. 6, the rising speed Va of the seed chuck 52 is decreased, and eventually the seed chuck 52 is lowered relative to the slider 56 (lifting jig 60) (Va becomes negative) in order to shift the weight of the single crystal C to the lifting jig 60.
In the above-described technique, the weight of the single crystal C acting on the seed chuck 52 is shifted to the lifting jig 60 while the speed Vb of the lifting jig 60 and the speed Va of the seed chuck 52 are changed in order to switch the pulling means. Therefore, when the lifting jig 60 deforms due to the weight of the crystal or slip occurs between the lifting jig 60 and the corrugated portion Ck, the speeds Va and Vb must be controlled in consideration of an increased number of factors including the above-described deformation and slip, so that the speed control becomes complex and fine adjustment becomes difficult to perform.
If fine adjustment of the speeds Va and Vb cannot be performed correctly, the growth rate of the single crystal varies, so that variations in the diameter of the crystal and other problems occur.
Therefore, there has been strong demand for a technique that can accurately control the diameter of a single crystal being pulled through simplified growth rate control, without use of the above-described complicated control.